Cavity resonators in good conductors can be fashioned so that only certain combinations of electric and magnetic fields can exist within the cavity. Such cavities are useful because they can filter out electromagnetic field energy at undesired frequencies.
A resonant cavity can be structured so that only particular modes of an electromagnetic field are utilized within the cavity. A dielectric post or metallic post is sometimes provided within the cavity, with its longitudinal axis extending out from a sidewall of the cavity, so as to be substantially perpendicular to the direction of flow of electromagnetic field energy within the cavity. Such posts impose behavior (expressed as boundary conditions) on the electric and magnetic fields, in addition to the behavior imposed by the electrically conducting metallic material of the cavity walls. The term dielectric post is used here to mean a dielectric (e.g. ceramic) puck (i.e. a short cylinder of ceramic material) held away from a wall of the cavity by a support; the longitudinal axis of the dielectric puck is substantially perpendicular to the direction of flow of electromagnetic field energy within the cavity.
Depending on the type of resonator, i.e. whether the post material is metallic or dielectric, one or another behavior is imposed on the electric and magnetic fields. If the material is metallic and the cavity is operating in transverse electric and magnetic field (TEM) mode, the electric field within the cavity, besides being normal (perpendicular) to every (electrically conducting) cavity wall, or vanishing at such a wall, must also be normal to the surface of the metallic post, or must vanish at the surface of the post. The magnetic field, on the other hand, has only an azimuthal non-zero component within the cavity, taking the lengthwise axis of the post to be the axis about which the azimuthal angle is measured. (Thus, the electric field is zero within the post and normal to every surface within the cavity, including the surface of the metallic post, while the magnetic field is also zero within the post but runs circumferentially around the post.)
If the post material is a dielectric, such as a ceramic, on the other hand, the cavity can resonate in a transverse electric (TE) mode, in particular the TE.sub.011 mode. In such a mode, in a cavity with a ceramic post (i.e. a ceramic puck plus a spacer) having a longitudinal axis extending away from a sidewall of the cavity, the electric field will be purely azimuthal with respect to the center line axis of the ceramic post and largest within the ceramic post, and because the walls of the cavity are metallic, will decrease in intensity away from the ceramic post, vanishing at the walls of the cavity. The magnetic field, on the other hand, is everywhere orthogonal (perpendicular) to the electric field and has a radial component proportional to the electric field (although 90.degree. out of phase). Thus, the magnetic field will be largest within the ceramic post and will have no azimuthal component (with respect to the axis of the ceramic post) anywhere in the cavity.
A filter based on a metallic resonator has different performance characteristics from a filter based on a dielectric (ceramic) resonator. In particular, ceramic resonators generally provide poor spurious performance compared to a metallic resonator, and a metallic resonator is usually less expensive. A ceramic resonator on the other hand is superior to a metallic resonator in its passband performance, due to the higher quality factor of a ceramic resonator. Thus, it is desirable to build filters using both kinds of cavity resonators, i.e. dissimilar cavities, and so to obtain a filter combining the better qualities of each kind of cavity resonator.
Unfortunately, as is evident from the above description of the electric and magnetic fields in the two different kinds of resonators, if a ceramic cavity is physically adjacent a metallic cavity, and no special structure is used to couple the two cavities, then the axis of the ceramic post in the ceramic cavity must be perpendicular to the axis of the metallic post in the metallic cavity (and also perpendicular to the direction of flow of energy from one end of the filter cavity to the other) so that the magnetic fields or the electric fields in the two cavities align. If this is not done, there can be no flow of energy between the cavities because the magnetic field and electric field in the second cavity can only exist in an orientation not possible in the first cavity.
The prior art, as shown in FIGS. 2-4, sometimes arranges physically adjacent cavities so that the possible magnetic field orientations in the two cavities have some mutual components (with respect to a single frame of reference). In FIGS. 2-4, a filter according to the prior art is shown made from a ceramic resonator 16 coupled by a coupling structure 18 to a metallic resonator 17, and having ports 25. The electromagnetic energy flows from one port through the cavity to the other port. The ceramic resonator 16 has a ceramic puck 11 spaced apart from a sidewall of the ceramic resonator cavity wall 20 using a support 19. The metallic resonator 17 includes a metallic post 12 and capacitive screw 13 (FIG. 2 only). The metallic post 12 is affixed to a wall 21 of the metallic resonator cavity, and the capacitive screw 13 is threaded through the opposite wall 22. With this relative arrangement of the posts 11 and 12, the magnetic field in the two cavities 16 and 17 is aligned, i.e. has some same non-zero components. Thus, the coupling structure 18, separating the two cavities 16 and 17 with a metallic wall 15 having an aperture 14, need only provide a direct path for the electromagnetic field from one cavity 16 or 17 to the other, because the magnetic field in one cavity is already partially aligned with the magnetic field in the other cavity.
This arrangement, although useful, has the drawback that the mechanical layout of one cavity fixes that of the physically adjacent cavity, and in the case of a multistage filter consisting of one or another combination of three or more cavities of dissimilar types, can lead to annoying complications.
The prior art uses other means of coupling dissimilar cavities besides mechanically orienting physically adjacent cavities. These other methods focus on aligning either the electric field, using a probe-to-probe coupling structure to draw the electric field from one cavity into an orientation suitable for the physically adjacent cavity, or aligning the magnetic field, using a loop-to-loop coupling structure. Besides these aligning-type coupling structures, the prior art uses a probe-to-loop coupling structure to have the electric field in one cavity produce a current in a loop extending into the physically adjacent cavity and so produce a magnetic field in the physically adjacent cavity oriented in a way suitable for the physically adjacent cavity by properly orienting the loop. These probe and loop structures are of use, however, only for relatively narrow bandwidth filters because the electric coupling they provide is relatively weak.
What is needed is a coupling structure for coupling dissimilar resonators that couples, relatively strongly, the dissimilar resonators without fixing the relative orientations of the dissimilar resonators.